The invention relates to a circuitry arrangement by means of which it is possible to suppress the frequency components which cause spurious response in the intermediate-frequency amplifier of a radio telephone receiver, the amplifier producing not only the output signal frequency but also harmonic frequencies of this frequency, in which case some harmonic frequency of the frequency component which causes spurious response is the same as the intermediate frequency.
Usually in radio telephones the received radio signal is directed from the antenna to a duplexer, from which it is directed via a first amplifier of the front end of the receiver part and a fixed bandpass filter to a first mixer, in which it is mixed with the local-oscillator frequency, and the intermediate frequency is filtered from the output results.
The intermediate-frequency signal is amplified before it is directed to a second mixer. For reasons of component costs and in order to simplify the basic construction of the receiver it would be advantageous to select a low intermediate frequency, but the spurious response caused by the slight non-linearity of the intermediate-frequency amplifier operating at the first intermediate frequency causes problems. A spurious response is produced because the intermediate-frequency amplifier, being a slightly non-linear circuit, as stated above, produces not only the amplified basic-frequency input signal but also multiple frequencies of this frequency. If a signal, the frequency of which is one-half of the intermediate frequency, arrives at the input of such an amplifier, there can be measured in the output of the amplifier also an intermediate-frequency signal which is to be regarded as a spurious response.
When the intermediate frequency is sufficient in relation to the reception band, at least greater than 1.times. the width of the reception band, the frequency components producing the spurious response can be filtered with fixed filters in the high-frequency parts of the receiver. For this reason the first intermediate frequency is usually selected so as to be sufficiently high, even though the components to be used are more difficult to manufacture and are thus expensive. For example, in the NMT radio telephone system the width of the reception band is 25 MHz and the intermediate frequency is over 50 MHz, for example 87 MHz. Spurious response does not always cause great harm, especially if the amplitude of the input signal which produces it is low, but in certain radio telephone systems the spurious response rejection required of a telephone is so great that the receiver will not operate in accordance with the specifications without a separate method for rejecting the spurious response caused by harmonic components.